In the low-frequency domain (\(100\mu \) Hz and 100 mHz), space-based interferometers such as the Laser Interferometer Space Antenna (LISA; [1]}), the interferometer TianQin (under design [2]}) and the proposed Taiji program ([3]}) will instead detect the GW signals from massive binary BH coalescences (in the range \(\sim 10^4-10^7 \, \rm M_\odot \) ), across cosmic ages (e.g., [4]}, [5]} and references therein).
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